U.S. patent application number 11/827420 was filed with the patent office on 2008-01-17 for reagent dosing pump.
Invention is credited to Michael P. Cooke.
Application Number | 20080014103 11/827420 |
Document ID | / |
Family ID | 37668198 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080014103 |
Kind Code |
A1 |
Cooke; Michael P. |
January 17, 2008 |
Reagent dosing pump
Abstract
A pump for pumping a liquid, the pump comprising an inlet, an
outlet, a pumping chamber for receiving the liquid from the inlet,
and an actuator arrangement operable between a first position and a
second position and arranged to pump the liquid from the pumping
chamber into the outlet, wherein the inlet and the pumping chamber
are in fluid communication with a supply passage when the actuator
arrangement is in the first position, and the supply passage
extends into or around the actuator arrangement so as to allow
transfer of heat from the actuator arrangement to the liquid.
Inventors: |
Cooke; Michael P.;
(Gillingham, GB) |
Correspondence
Address: |
DELPHI TECHNOLOGIES, INC.
M/C 480-410-202, PO BOX 5052
TROY
MI
48007
US
|
Family ID: |
37668198 |
Appl. No.: |
11/827420 |
Filed: |
July 11, 2007 |
Current U.S.
Class: |
417/410.1 |
Current CPC
Class: |
Y02T 10/24 20130101;
F01N 3/2066 20130101; Y02T 10/12 20130101; F01N 2610/1433 20130101;
F01N 2610/02 20130101; F01N 2610/10 20130101; F04B 17/048
20130101 |
Class at
Publication: |
417/410.1 |
International
Class: |
F04B 17/00 20060101
F04B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2006 |
EP |
06253638.8 |
Claims
1. A pump for pumping a liquid, the pump comprising: an inlet; an
outlet; a pumping chamber for receiving the liquid from the inlet;
and an actuator arrangement operable between a first position and a
second position and arranged to pump the liquid from the pumping
chamber into the outlet; wherein the inlet and the pumping chamber
are in fluid communication with a supply passage when the actuator
arrangement is in the first position, and the supply passage
extends into or around the actuator arrangement so as to allow
transfer of heat from the actuator arrangement to the liquid.
2. A pump according to claim 1, wherein the actuator arrangement is
disposed substantially between the inlet and the outlet.
3. A pump according to claim 1, further comprising a delivery valve
operable between a closed position and an open position and
arranged to restrict the flow of liquid from the pumping chamber to
the outlet when the delivery valve is in the closed position.
4. A pump according to claim 1, wherein a fluid communication path
is provided between the pumping chamber and the supply passage by
one or more filling ports.
5. A pump according to claim 4, wherein the or each filling port is
blocked by the actuator arrangement when the actuator arrangement
is in the second position.
6. A pump according to claim 1, wherein the actuator arrangement
comprises a plunger arranged to move in response to switching of
the actuator arrangement between the first position and the second
position.
7. A pump according to claim 6, wherein the plunger is arranged to
cause a change in volume of the pumping chamber when the actuator
arrangement is switched between the first position and the second
position.
8. A pump according to claim 7, wherein the actuator arrangement
further comprises a stopper carried on the plunger.
9. A pump according to claim 8, wherein the position of the stopper
with respect to the plunger is arranged to be adjustable so as to
influence the change in volume of the pumping chamber that occurs
when the actuator arrangement is switched between the first
position and the second position.
10. A pump according to claim 8, further comprising a lift stop and
wherein the stopper is arranged to abut the lift stop when the
actuator arrangement is in the first position.
11. A pump according to claim 10, wherein the position of the lift
stop with respect to the actuator arrangement is arranged to be
adjustable so as to influence the distance through which the
plunger moves when the actuator arrangement is switched between the
first position and the second position.
12. A pump according to claim 1, wherein the actuator arrangement
comprises a solenoid actuator comprising a solenoid coil, and the
supply passage extends into or around the solenoid coil.
13. A pump according to claim 12, wherein the solenoid coil defines
an axis and the supply passage is arranged so that, in use, the
direction of flow of liquid in the supply passage is substantially
parallel to the axis of the solenoid coil.
14. A pump according to claim 12, further comprising a pole element
and a coil former to carry the solenoid coil, wherein the coil
former is disposed around at least a part of the pole element and
the supply passage is defined partly by a surface of the pole
element and partly by a surface of the coil former.
15. A pump according to claim 1, wherein the actuator arrangement
is arranged so that, in use, the temperature of the actuator
arrangement increases upon energisation of the actuator arrangement
so as to heat the supply passage.
16. A pump according to claim 1, wherein the liquid is a reagent
for selective catalytic reduction.
17. A dosing device comprising a pump according to claim 1.
18. An exhaust system comprising a dosing device according to claim
17.
19. A method of cooling a pump for dispensing a liquid in a gas
flow, the pump comprising an inlet, a pumping chamber and an
actuator arrangement; the method comprising: supplying the liquid
to the inlet; transferring the liquid to the pumping chamber by
passing the liquid into or around the actuator arrangement; and
pumping the liquid from the pumping chamber to the outlet so as to
carry heat away from the actuator arrangement.
20. A method of melting frozen liquid in the pump according to
claim 1; the method comprising: energising the actuator arrangement
so as to cause heating of the actuator arrangement and melting of
the frozen liquid.
21. A pump for pumping a liquid, the pump comprising: an inlet; an
outlet; a pumping chamber for receiving the liquid from the inlet;
a pole element; an actuator arrangement comprising a solenoid
actuator comprising a coil former carrying a solenoid coil, the
actuator arrangement being operable between a first position and a
second position and arranged to pump the liquid from the pumping
chamber into the outlet; wherein the inlet and the pumping chamber
are in fluid communication with a supply passage when the actuator
arrangement is in the first position, and the supply passage,
defined partly by a surface of the pole element and partly by a
surface of the coil former extends into or around the actuator
arrangement so as to allow transfer of heat from the actuator
arrangement to the liquid.
Description
[0001] This invention relates to a device for use in a system for
dosing exhaust gases with reagent, for example to reduce emissions
of harmful substances to the atmosphere. In particular, the
invention relates to a pump for dispensing reagent.
[0002] It is well known that exhaust gases from internal combustion
engines contain substances which are harmful to the environment and
which can pose a threat to public health. For many years, a
sustained effort has been made within the automotive industry to
reduce the release to the atmosphere of harmful substances carried
in exhaust gases, both by modifying the combustion process itself
to give a reduced yield of harmful combustion products, and by
treating the exhaust gases before their emission into the
atmosphere, for example by providing a catalyst to induce chemical
breakdown of the harmful constituents into benign compounds.
[0003] One class of harmful exhaust gas constituents comprises the
oxides of nitrogen, with the generic chemical formula NO.sub.x,
where x typically ranges from 0.5 to 2.5. Nitrogen oxides
contribute to the formation of ground-level ozone, nitrate
particles and nitrogen dioxide, all of which can cause respiratory
problems. Furthermore, nitrogen oxides can lead to the formation of
acid rain, and nitrous oxide (N.sub.2O) in particular is a
greenhouse gas and contributes to the destruction of the ozone
layer. It is therefore desirable to reduce the emission of nitrogen
oxides into the atmosphere, and furthermore, new vehicles must
comply with increasingly stringent limits on the acceptable levels
of NO.sub.x emissions.
[0004] In certain circumstances, NO.sub.x emissions can be reduced
by conventional exhaust gas catalysis, for example in a three-way
catalyst comprising immobilised powders of platinum, palladium and
rhodium. However, in diesel or lean-burn petrol combustion engines,
a high concentration of oxygen is present in the exhaust gas, and
this oxygen inhibits the catalysed breakdown of the nitrogen oxides
in conventional systems. Consequently, a need has arisen for an
alternative strategy to limit NO.sub.x emissions.
[0005] One strategy, known as selective catalytic reduction or SCR,
involves the introduction of a reagent comprising a reducing agent,
typically a liquid ammonia source such as an aqueous urea solution,
into the exhaust gas stream. The reducing agent is injected into
the exhaust gas upstream of an exhaust gas catalyst, known as an
SCR catalyst, typically comprising a mixture of catalyst powders
such as titanium oxide, vanadium oxide and tungsten oxide
immobilised on a ceramic honeycomb structure. Nitrogen oxides in
the exhaust gas undergo a catalysed reduction reaction with the
ammonia source on the SCR catalyst, forming gaseous nitrogen and
water. An example of such a system is described in International
Patent Application No. WO 2004/111401 A.
[0006] Although aqueous urea is a convenient and cost-effective
source of ammonia for SCR systems, the maximum temperature at which
it can be used is somewhat limited. Urea crystals tend to
precipitate when the temperature of the solution is greater than
approximately 70.degree. C. Precipitation is undesirable because
the precipitates can cause blockages in the delivery system, for
example in the small-diameter outlets typically provided in an
atomising nozzle. In addition, the formation of precipitates alters
the concentration of the remaining solution, so that the effective
quantity of ammonia delivered to the exhaust flow becomes
uncertain. This could lead to inefficient catalysis and an
insufficient reduction in NO.sub.x emissions.
[0007] If aqueous urea is to be used effectively as a reagent in
SCR, the system provided for dosing the exhaust gases with reagent
should ideally be arranged to ensure that the temperature of the
urea solution does not exceed the temperature at which
precipitation occurs. However, the reagent must be discharged into
the stream of hot exhaust gases, which are typically at a
temperature of around 400.degree. C. at the point where the reagent
enters the exhaust gas stream. The reagent will therefore almost
inevitably reach a temperature in excess of that at which solid
precipitates begin to form.
[0008] In the Applicant's United States Patent Application No.
US2004/0093856, a solenoid-operated reagent dosing pump is
described. Because this pump can generate high reagent pressures,
it is able to blow precipitates through an outlet nozzle. In this
way, any solid particles that form due to overheating of the
reagent can be forced out of the dosing system and into the exhaust
gas stream and are prevented from blocking the flow of reagent.
Furthermore, the use of a high-pressure solenoid pump allows the
delivery of small quantities of reagent at high frequencies, with
the result that the mixture of exhaust gas and reagent flowing on
to the SCR catalyst has a more uniform temperature and composition
in comparison to other systems, in which larger quantities of
reagent are delivered at lower frequencies. This improves the
efficiency of the reduction reactions occurring at the catalyst,
because the temperature and gas composition can be better
maintained at their optimum levels for reaction.
[0009] Although use of a solenoid pump offers significant
advantages for reagent dosing devices, one potential drawback
arises from the sensitivity of the solenoid to temperature. The
efficiency of the solenoid, often expressed as the ratio of the
mechanical power output to the electrical power input, decreases as
the temperature of the solenoid increases. This decrease in
efficiency is due in part to the increase in resistance of the coil
with temperature. When used in a reagent dosing system, the
solenoid tends to heat up due to the proximity of the pump to the
hot exhaust system, and due to the resistive heating of the coil.
The temperature in the vicinity of the solenoid is also relatively
high, which that dissipation of heat from the solenoid into its
surroundings is limited.
[0010] Against this background, it would be desirable to provide a
reagent dosing device for use in an exhaust gas dosing system which
overcomes or alleviates the abovementioned problems.
[0011] According to a first aspect of the present invention, there
is provided a reagent dosing device comprising a pump for pumping a
liquid, the pump comprising an inlet, an outlet, a pumping chamber
for receiving the liquid from the inlet, and an actuator
arrangement operable between a first position and a second position
and arranged to pump the liquid from the pumping chamber into the
outlet, wherein the inlet and the pumping chamber are in fluid
communication with a supply passage when the actuator arrangement
is in the first position, and the supply passage extends into or
around the actuator arrangement so as to allow transfer of heat
from the actuator arrangement to the liquid.
[0012] Because heat is transferred from the actuator arrangement to
the liquid on passage of the liquid through the pump, heat is
carried away from the actuator arrangement and hence from the pump
when the heated reagent is dispensed from the pump by way of the
outlet. In this way, the liquid acts to cool the actuator
arrangement of the pump, thereby allowing the actuator arrangement
to operate at an improved efficiency when compared to a pump in
which no significant cooling occurs.
[0013] The actuator arrangement may be disposed substantially
between the inlet and the outlet so that the liquid can pass
conveniently into or around the actuator arrangement.
[0014] The pump may further comprise a delivery valve operable
between a closed position and an open position and arranged to
restrict the flow of liquid from the pumping chamber to the outlet
when the delivery valve is in the closed position. The provision of
a delivery valve allows the pressure of liquid in the pumping
chamber to build up before the liquid is passed into the outlet, so
that a high pressure of the liquid is obtained in the outlet. The
delivery valve may be arranged so that the flow of liquid from the
pumping chamber to the outlet substantially ceases when the
delivery valve is in the closed position. Alternatively, the
delivery valve may be arranged so that, when the delivery valve is
in the closed position, the flow of liquid from the pumping chamber
to the outlet can occur at a reduced rate relative to when the
delivery valve is in an open position.
[0015] A fluid communication path may be provided between the
pumping chamber and the supply passage by one or more filling
ports. For example, filling ports may be provided in the actuator
arrangement. In one arrangement, the fluid communication path is
blocked by the actuator arrangement when the actuator arrangement
is in the second position.
[0016] Conveniently, the actuator arrangement comprises a plunger
arranged to move in response to switching of the actuator
arrangement between the first position and the second position. The
plunger may be arranged to cause a change in volume of the pumping
chamber when the actuator arrangement is switched between the first
position and the second position. For example, when the actuator
arrangement is switched from the first position to the second
position, the plunger may cause a decrease in the volume of the
pumping chamber, and hence an increase in the liquid pressure
within the pumping chamber so as to cause pumping of the liquid
from the pumping chamber.
[0017] The actuator arrangement may further comprise a stopper
carried on the plunger. The position of the stopper with respect to
the plunger may be arranged to be adjustable so as to influence the
change in volume of the pumping chamber that occurs when the
actuator arrangement is switched between the first position and the
second position. Hence, the volume of liquid pumped from the
pumping chamber when the actuator arrangement is switched from the
first position to the second position can be adjusted, for example
during manufacture of the device.
[0018] Similarly, the pump may further comprise a lift stop, and
the stopper may be arranged to abut the lift stop when the actuator
arrangement is in the first position. The position of the lift stop
with respect to the actuator arrangement may be arranged to be
adjustable so as to influence the distance through which the
plunger moves when the actuator arrangement is switched between the
first position and the second position. In this way, the
acceleration of the plunger can be controlled to influence the
change in pressure with time of the liquid leaving the pump by the
outlet, for example during manufacture of the device.
[0019] The actuator arrangement comprises a solenoid actuator
comprising a solenoid coil, and the supply passage extends into or
around the solenoid coil. In one such arrangement, the solenoid
coil defines an axis and the direction of flow of liquid in the
supply passage is substantially parallel to the axis. The pump may
further comprise a pole element and a coil former to carry the
solenoid coil, and the coil former may be disposed around at least
a part of the pole element so that the supply passage may be
defined partly by a surface of the pole element and partly by a
surface of the coil former.
[0020] When a solenoid actuator is employed along with a plunger
carrying a stopper, the stopper can conveniently be an armature of
the solenoid actuator.
[0021] Because the actuator arrangement is cooled by transfer of
heat from the actuator arrangement to the liquid, in use, a
solenoid actuator deployed in the present invention will remain at
a lower temperature than if significant transfer of heat from the
actuator to the liquid did not occur. As a consequence, the
efficiency of the solenoid actuator is optimised.
[0022] Although a solenoid actuator is particularly suitable for
use in the present application, an alternative actuator arrangement
such as a piezoelectric or hydraulic actuator could be employed in
the present invention. Again, the cooling effected by the transfer
of heat from the actuator arrangement to the liquid serves to
improve the efficiency of the actuator arrangement, when compared
to a pump in which significant cooling of the actuator does not
occur.
[0023] The present invention also contemplates in a second aspect a
method of cooling a pump for dispensing a liquid in a gas flow, the
pump comprising an inlet, a pumping chamber and an actuator
arrangement; the method comprising supplying the liquid to the
inlet; transferring the liquid to the pumping chamber by passing
the liquid into or around the actuator arrangement; and pumping the
liquid from the pumping chamber to the outlet so as to carry heat
away from the actuator arrangement.
[0024] The pump of the present invention is ideally suited to
applications in which the liquid is a reagent for selective
catalytic reduction (SCR). The invention therefore extends to a
dosing device comprising a pump in accordance with any of the above
described pumps. The dosing device may, for example, further
comprise a dispensing for dispensing the liquid into a gas flow. A
connector to provide a fluid communication path between the outlet
of the pump and the dispensing may also be provided in the dosing
device. The dispensing may comprise a dispenser having a nozzle and
a nozzle valve to control the flow of liquid through the dispenser.
Furthermore, the invention also extends to an exhaust system
comprising a dosing device as previously described.
[0025] The liquid for selective catalytic reduction may be an
aqueous solution of urea. The lower temperature limit for use of
the urea solution is relatively modest. For example, at a typical
concentration (32%), the urea solution freezes at -11.5 C, which is
well above the minimum ambient temperature that might routinely be
expected in many countries. Of course, the solution cannot be
pumped or sprayed when it is frozen.
[0026] The actuator arrangement may be arranged so that, in use,
the temperature of the actuator arrangement increases upon
energisation of the actuator arrangement so as to heat the supply
passage. In this way, should liquid freeze within the pump, heat is
supplied to the frozen liquid in the supply passage to melt the
frozen liquid, therefore allowing the pump to operate. To this end,
according to a third aspect of the invention, there is provided a
method of melting frozen liquid in a pump as previously described,
the method comprising energising the actuator arrangement so as to
cause heating of the actuator arrangement and melting of the frozen
liquid.
[0027] Freezing of liquid often causes a volume increase. For
example, the volume of a typical urea solution increases by
approximately 10% on freezing. So as to avoid the development of
significant stresses within the pump should the liquid freeze, the
supply passage may have a relatively narrow cross-section in at
least one dimension. For example, the supply passage may be an
annular chamber with a thin radial dimension so as to avoid the
development of significant radial stresses.
[0028] According to a fourth aspect of the invention, there is
provided a pump suitable for dispensing a liquid into an exhaust
gas flow of an internal combustion engine, the pump comprising an
inlet, an outlet and an actuator arrangement, wherein the inlet,
the outlet and the actuator arrangement are arranged substantially
along a common axis so that, in use, liquid passes through the pump
and the actuator arrangement substantially in a direction parallel
to the common axis.
[0029] The pump according to the fourth aspect of the invention is
advantageous because the liquid passes along a largely straight,
non-tortuous path. For example, this allows the liquid to pass
through the pump with little or no impediment due to turbulent flow
at corners or kinks in the flow path of the liquid. Energy need not
be expended in overcoming such impediment, therefore the efficiency
of the pump in this respect is maximised. Furthermore, the pump is
of a compact design and is suitable for mounting directly in the
flow path of liquid from a source of liquid to a dispenser such as
a nozzle. The pump is also convenient to manufacture. For instance,
many of the components of the pump could be circular in
cross-section and could be fabricated by machining on a lathe, with
little additional machining required to create the final
components. Such a fabrication process offers a low-cost
manufacturing route which can be readily automated.
[0030] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying
drawings, in which:
[0031] FIG. 1 is a cross-sectional view of a dosing device
according to the present invention, mounted in an exhaust pipe;
[0032] FIG. 2 is a cross-sectional view at larger scale of the pump
of the dosing device of FIG. 1;
[0033] FIG. 3 is a cross-sectional view at larger scale of the
dispenser of the dosing device of FIG. 1;
[0034] FIG. 4 is a cross-sectional view at larger scale of the pump
of the dosing device of FIG. 1, when energised;
[0035] FIG. 5 is a cross-sectional view at larger scale of the
dispenser of the dosing device of FIG. 1, when energised;
[0036] FIG. 6 is a cross-sectional view of a dosing device
according to a second embodiment of the present invention, mounted
in an exhaust pipe.
[0037] In this specification, the terms `downstream` and `upstream`
refer to the direction of reagent flow through the device during
dispensing of the reagent into the gas flow during normal use. For
example, `downstream` is leftwards in FIG. 2 and rightwards in FIG.
3.
[0038] Referring to FIG. 1, a dosing device comprises a pump 20, a
connecting pipe 22, and a dispenser 24. The connecting pipe 22 is
generally semicircular and is connected at a first end to the pump
20 by way of a pump connector 26. The dispenser 24 is located at a
second end of the connecting pipe 22. In use, the device is mounted
to an exhaust pipe 28 of an internal combustion engine (not shown).
The dispenser 24 is disposed within a flow of exhaust gases within
the exhaust pipe 28, while the pump 20 is disposed outside the
exhaust pipe 28.
[0039] A mounting bracket 30 is provided to attach the pump 20 to
the exhaust pipe 28. The mounting bracket 30 comprises an
`L`-shaped plate having a first portion 32 (horizontal in FIG. 1)
and a second portion 34 (vertical in FIG. 1) perpendicular to the
first portion 32. The first portion 32 of the mounting bracket 30
is attached to the exhaust pipe 28, for example by a welded joint.
The second portion 34 of the mounting bracket 30 contains an
aperture 36 and spring clips 38 disposed around the aperture, for
example in the form of a star washer. The pump 20 is accommodated
within the aperture 36 and is retained in the mounting bracket 30
by the spring clips 38.
[0040] The connecting pipe 22 comprises a tube 40 having a bore 42
through which reagent can pass. The tube 40 is capable of
accommodating reagent at high pressure. The tube 40 is received
within a jacket 44 for the connecting pipe 22 which defines a
compartment 46 between the tube 40 and the jacket 44. The jacket 44
is sealed to the tube 40 at a first seal 48 at the outlet or pump
connector 26 and at a second seal 50 at the dispenser 24 so that
the compartment 46 defines an enclosed volume. The compartment 46
is evacuated to limit the transfer of heat from the hot exhaust
gases within the exhaust pipe 28 to the reagent in the bore 42 of
the tube 40, in use, so as to prevent overheating of the
reagent.
[0041] The connecting pipe 22 passes through a port 52 in a wall of
the exhaust pipe 28. The port 52 is cylindrical, and has a
surrounding collar 54 on the external surface of the exhaust pipe
28. The collar 54 is shaped to define a planar mounting face 56. An
annular flange 58 is carried on the jacket 44 of the connecting
pipe 22. The flange 58 is pressed towards the mounting face 56 by a
clamping ring 60, and a sealing washer 62 is clamped between the
flange 58 and the mounting face 56 to form a gas-tight seal.
Threaded studs (not shown) are carried on the collar 54 and pass
through complementary holes (not shown) in the sealing washer 62,
flange 58 and clamping ring 60. The clamping ring 60 is held on the
exhaust pipe 28 by way of internally-threaded nuts 64 screwed onto
the studs.
[0042] Referring to FIG. 2, the pump 20 comprises an inlet being an
inlet connector 66, a casing 68, and a pole element 70. The casing
68 comprises a first cylindrical portion 72 and a second
cylindrical portion 74, the first portion 72 having smaller
diameter than the second portion 74 to define a shoulder 76 of the
casing 68. The inlet connector 66 is generally tubular and
comprises a downstream portion 78 and an upstream portion 80. The
downstream portion 78 has a larger diameter than the upstream
portion 80 and carries external threads (not shown). The first
portion 72 of the casing 68 carries complementary internal threads
(not shown) to mate with the external threads of the inlet
connector 66.
[0043] The upstream portion 80 of the inlet connector 66 comprises
a bore defining an inlet passage 82. The downstream portion 78 of
the inlet connector 66 includes a bore with larger diameter than
the inlet passage 82 to define a filter chamber 84. The filter
chamber 84 accommodates a reagent filter 86, comprising a fine
woven metallic or plastic mesh shaped into a thimble-shape. A
perimeter rim 88 of the reagent filter 86 is attached to a lift
stop 90 comprising an annular ring or washer. The lift stop 90 is
attached to the downstream end of the inlet connector 66 and is
accommodated partly within the downstream portion 78 of the inlet
connector 66 and partly within the first portion 72 of the casing
68.
[0044] The pole element 70 comprises a generally cylindrical inner
pole piece 92, an outwardly-directed flange 94 and a central
tubular land or projection 96 situated downstream of the flange 94.
The downstream, outermost edge 98 of the flange 94 is bevelled.
[0045] The second portion 74 of the casing 68 has substantially
uniform wall thickness except for a region at the downstream end of
the casing 68, where the wall thickness of the casing 68 is reduced
to define an annular internal groove 100 bounded by an internal
shoulder 102. The flange 94 of the pole element 70 is accommodated
within the groove 100 of the casing 68. The downstream end of the
casing 68, adjacent to the groove 100, is bent over the bevelled
edge 98 of the flange 94, for example by crimping or pressing
during manufacture, so as to hold the flange 94 against the
shoulder 102 of the casing 68.
[0046] The second portion 74 of the casing 68 houses a solenoid
coil 104 wound on to a coil former 106. The coil former 106 is a
ring with a generally `U`-shaped radial cross section. A first arm
of the `U` defines a first face 108 of the coil former 106 and a
second arm of the `U` defines a second face 110 of the coil former
106. The coil 104 is disposed between the first and second faces
108, 110 of the coil former 106. The coil former 106 is disposed
around the inner pole piece 92 of the pole element 70, and a supply
passage comprising an annular chamber 112 is defined between the
coil former 106 and the inner pole piece 92.
[0047] The first face 108 of the coil former 106 carries an annular
groove 114, in which a first o-ring 116 is provided. The first
o-ring 116 is arranged to create a seal between the shoulder 76 of
the casing 68 and the first face 108 of the coil former 106. The
second face 110 of the coil former 106 also carries an annular
groove 118, adjacent to the inside of the coil former 106, in which
a second o-ring 120 is provided. The second o-ring 120 is arranged
to create a seal between the flange 94 of the pole element 70 and
the second face 110 of the coil former 106. The first and second
o-rings 116, 120 are made from a heat-resistant rubber and are
arranged to elastically deform during assembly of the pump 20.
[0048] The coil 104 is in electrical communication with a power
supply (not shown) by way of a supply cable 122. The power supply
is capable of supplying a variable current to the coil 104 so as to
induce a variable magnetic field around the coil 104. The
arrangement of components within the pump 20 will first be
described for the situation where no current is supplied to the
coil 104, so that no magnetic field is present around the coil
104.
[0049] The pole element 70 includes an axial bore 124. A plunger
126 is slidably accommodated within the bore 124. A disc-shaped
armature 128 is attached by an interference fit to the plunger 126
near to an upstream end 130 of the plunger 126. The armature 128 is
a clearance fit in the first portion 72 of the casing 68. A
delivery valve 132 is provided downstream of the plunger 126. The
bore 124 of the pole element 70 comprises an enlarged diameter
portion downstream of the plunger. The enlarged diameter portion
defines a delivery valve chamber 134 and a shoulder or seating
surface 136 of the delivery valve 132. A delivery valve element
138, comprising a disc, is provided within the delivery valve
chamber 134. An orifice 140 with small diameter is provided through
the thickness of the delivery valve element 138.
[0050] The pump connector 26 is defined by an end region 142 of the
tube 40 of the connecting pipe 22 with greater wall thickness than
that of the tube 40 distant from the pump connector 26. An end
region 142 of the tube 40 has an enlarged outer diameter to meet
the jacket 44 of the connecting pipe 22 at the first seal 48. A
portion of the end region 142 emergent from the jacket 44 is
accommodated within a tubular projection 96 of the pole element 70.
The bore of the pump connector 26 has an increased diameter in a
portion of the end region 142 adjacent to the delivery valve
chamber 134 so as to define a shoulder 144 of the tube 40 and a
delivery spring chamber 146.
[0051] A delivery valve spring 148, comprising a compression
spring, is accommodated within the delivery spring chamber 146. The
spring 148 acts upon the shoulder 144 of the tube 40 and the
delivery valve element 138 to bias the delivery valve element 138
against the seating surface 136 of the pole element 70 to close the
delivery valve 132. The bore 42 of the tube 40 of the connecting
pipe 22 is in communication with the delivery valve chamber 134 by
way of radial channels 150 formed in the end of the pump connector
26.
[0052] The bore 124 of the pole element 70, the delivery valve
element 138 and a downstream end 152 of the plunger 126 together
define a pumping chamber 154 downstream of the plunger 126. The
pumping chamber 154 is in communication with the annular chamber
112 by way of filling ports 156 comprising radial passages provided
in the inner pole piece 92 of the pole element 70.
[0053] The bore 124 of the pole element 70 is enlarged in a region
adjacent to the armature 128 so as to define a shoulder 158 within
the bore and a return spring chamber 160. A return spring 162
comprising a compression spring is disposed within the return
spring chamber 160. The return spring 162 acts upon the shoulder
158 of the bore 124 and the armature 128 to bias the armature 128
against the lift stop 90 so that the armature 128 acts as a stopper
for the plunger.
[0054] Referring to FIG. 3, the dispenser 24 comprises a nozzle 163
defined by a region of the tube 40 of the connecting pipe 22 shaped
into a conical frustum. A rim 164 of the nozzle 163 meets with the
jacket 44 of the connecting pipe 22 to form the second seal 50. The
nozzle 163 includes a nozzle bore 166 with a smaller diameter than
the diameter of the bore 42 of the tube 40 distant from the
dispenser 24.
[0055] The dispenser 24 further comprises a nozzle valve 168,
including a nozzle valve element 170 accommodated within the nozzle
bore 166. The nozzle valve element 170 comprises a piston 172 and a
shaft 174 upstream of the piston 172. The piston 172 is generally a
close clearance fit in the nozzle bore 166, so that the nozzle
valve element 170 may slide within the nozzle bore 166. An annular
groove 176 is provided in a downstream portion of the piston 172
adjacent to a tip 178 of the nozzle 163 to define a nozzle chamber
180 between the piston 172 and the nozzle bore 166. An end of the
piston 172 adjacent to the tip 178 of the nozzle 163 is flared
outwards to define a valve surface 182 of the piston 172 having a
diameter greater than the nozzle bore 166. The nozzle bore 166
adjacent to the tip 178 of the nozzle 163 is flared to define a
sealing surface 184 of the nozzle 163 complementary to the shape of
the valve surface 182 of the piston 172.
[0056] The end of the piston 172 distant from the tip 178 of the
nozzle 163 is attached to the shaft 174 of the nozzle valve element
170. A bush 186 is provided within the nozzle bore 166, and the
shaft 174 is a clearance fit within the bush 186 to define a
clearance 187. The shaft 174 extends into the tube 40 of the
connecting pipe 22. A retaining collar 188 is carried on and fixed
to the shaft 174 close to the end of the shaft 174 distant from the
piston 172. A nozzle spring 190, comprising a compression spring,
is disposed between the retaining collar 188 and the bush 186 so as
to bias the valve surface 182 of the piston 172 against the sealing
surface 184 of the nozzle 163 to close the nozzle valve 168.
[0057] Helical grooves 192 are provided on an upstream portion of
the piston 172 to define helical passages between the piston 172
and the nozzle bore 166. The nozzle chamber 180 is therefore in
communication with the bore 42 of the tube 40 by way of the helical
passages and the clearance 187 between the shaft 174 and the bush
186.
[0058] In use, the pump 20 is connected to a reagent supply (not
shown) by way of the inlet passage 82 of the inlet connector 66,
and the internal spaces or passages within the pump 20, the
connecting pipe 22 and the dispenser 24 are filled with reagent. As
shown in FIG. 2, reagent can flow from the inlet passage 82 through
the reagent filter 86, which serves to filter solid particles such
as precipitates out of the reagent flow. The lift stop 90
incorporates radial passages 194 to allow reagent to pass between
the lift stop 90 and the armature 128 when the armature 128 is
biased against the lift stop 90. Reagent can flow into the pumping
chamber 154, by way of a clearance defined between the armature 128
and the casing 68, the annular chamber 112 defined between the
inner pole piece 92 and the coil former 106, and the filling ports
156.
[0059] In order to dispense reagent, a current is passed through
the coil 104 to energise the coil 104 and induce a magnetic field
around the coil 104. This causes reagent to be dispensed from the
pump 20 into the bore 42 of the connecting pipe 22 and expelled
from the dispenser 24 as will now be described with reference to
FIGS. 4 and 5.
[0060] As shown in FIG. 4, when a current flows through the coil
104, an electromagnetic circuit arises which encompasses the inner
pole piece 92, the armature 128 and the casing 68, which functions
as an outer pole piece. The components in the electromagnetic
circuit define a electromagnetic structure or actuator arrangement
and are arranged so that the armature 128 experiences a magnetic
force opposed to the biasing force of the return spring 162. When
the magnetic force is strong enough to overcome the biasing force
of the return spring 162, the armature 128 moves towards the inner
pole piece 92.
[0061] Movement of the armature 128 drives a pumping stroke of the
plunger 126 by causing the plunger 126 to move in a downstream
direction within the bore 124 of the pole element 70. The plunger
126 moves past the filling ports 156 to close the filling ports 156
and substantially prevent further passage of reagent into the
pumping chamber 154. The volume of the pumping chamber 154 is
reduced due to the movement of the plunger 126, so that the
pressure of the reagent in the pumping chamber 154 increases. The
reagent imparts a force on the delivery valve element 156 in a
direction opposed to the biasing force imparted on the delivery
valve element 156 by the delivery valve spring 148.
[0062] When the pressure of the reagent in the pumping chamber 154
exceeds a threshold value, the force acting on the delivery valve
element 156 due to the reagent becomes sufficient to overcome the
biasing force of the delivery valve spring 148 and the delivery
valve element 156 moves away from the seating surface 136 of the
pole element 70 to open the delivery valve 132. Reagent in the
pumping chamber 154 can then flow around the delivery valve element
156 and into the bore 42 of the connecting pipe 22, by way of the
radial channels 150 of the pump connector 26. The pressure of
reagent in the bore 42 of the connecting pipe 22 therefore
increases.
[0063] Referring to FIG. 5, the increase in pressure of the reagent
in the bore 42 of the connecting pipe 22 is experienced in the
nozzle chamber 180 of the dispenser 24. The pressure of the reagent
in the nozzle chamber 180 acts upon the valve surface 182 of the
piston 172 so as to impart a force on the nozzle valve element 170
opposed to the biasing force of the nozzle spring 190. When the
pressure of reagent exceeds a threshold value, the force acting on
the nozzle valve element 172 due to the reagent pressure is
sufficient to overcome the biasing force of the nozzle spring 190
and the nozzle valve element 170 moves in a downstream direction
and into an open position to create a clearance 196 between the
valve surface 182 of the piston 172 and the seating surface 184 of
the nozzle 163, so that the nozzle valve 168 becomes open for
dispensing of reagent into the exhaust gas flow.
[0064] When the nozzle valve 168 is open, reagent is expelled from
the dispenser 24 by way of the clearance 196. The size of the
clearance 196 and the shape of the seating surface 184 and valve
surface 182 can be adapted so that, upon expulsion of the reagent,
the reagent is atomised into a fine spray to aid dispersion and
mixing of the reagent within the exhaust gases in the exhaust pipe
28.
[0065] As will now be described, when the plunger 126 of the pump
20 reaches the end of its pumping stroke, pressure changes take
place within the dosing device so that the expulsion of reagent
through the dispenser 24 stops. The plunger 126 reaches the end of
its pumping stroke when the armature 128 meets the inner pole
element 92. Because the delivery valve element 156 is unseated from
the seating surface 136 of the pole element 70, the pressure in the
pumping chamber 154 decays as reagent flows out of the pumping
chamber 154. The force acting on the delivery valve element 156 due
to the reagent in the pumping chamber 154 therefore decays. When
the force applied by the reagent falls below the opposing force
applied to the delivery valve element 156 by the delivery valve
spring 148, the delivery valve element 156 closes against the
seating surface 136 of the pole element 70 to close the delivery
valve 132.
[0066] The pressure of the reagent in the bore 42 of the connecting
pipe 22 also decays as the reagent flows through the dispenser 24
and into the exhaust pipe 28. Hence, the pressure of the reagent in
the nozzle chamber 180 also decays, and the force acting on the
valve surface 182 of the piston 172 to maintain the clearance 196
diminishes. Once the force acting on the valve surface 182 of the
piston 172 due to the reagent falls below the restoring force
acting on the nozzle valve element 170 due to the nozzle spring
190, the nozzle valve element 170 closes to seat the valve surface
182 of the piston 172 against the seating surface 182 of the nozzle
163 to close the nozzle valve 168. Reagent discharge from the
dispenser 24 therefore ceases.
[0067] Upon opening of the nozzle valve 168, reagent flows through
the helical passages defined by the helical grooves 192 of the
piston 172. The helical passages cause the reagent to leave the
dispenser 24 with a swirling motion, which enhances atomisation of
the reagent. In addition, the reagent imparts a torque on the
piston 172 as the reagent flows through the helical passages, so
that the piston 172 rotates within the dispenser 24. When the
nozzle valve 168 closes, therefore, the valve surface 182 is
presented to the seating surface 184 of the nozzle 163 at a
different angular orientation. Such an arrangement prevents uneven
wear of the valve surface 182 and the seating surface 184 which
could lead to unwanted leakage of reagent from the dispenser 24
when the nozzle valve 168 is closed. In addition, rotation of the
piston 172 encourages the removal of solid deposits from the
dispenser 24, for example by dislodging solid particles from the
nozzle bore 166, so as to minimise the possibility of the dispenser
24 becoming blocked by solid deposits.
[0068] When the current flow through the coil 104 is switched off,
the magnetic field around the coil 104 diminishes. The magnetic
force acting on the plunger 126, by way of the armature 128,
diminishes and can no longer overcome the biasing force of the
return spring 162. The action of the return spring 162 on the
armature 128 drives a return stroke of the plunger 126, in which
the plunger 126 returns to its initial position in which the
armature 128 rests against the lift stop 90. The volume of the
pumping chamber 154 increases and communication between the filling
ports 156 and the pumping chamber 154 is restored. Reagent can flow
through the filling ports 156 into the pumping chamber 154, to
replenish the reagent in the pumping chamber 154 in preparation for
the next pumping stroke.
[0069] The orifice 140 in the delivery valve element 156 provides a
communication through the delivery valve 132 between the pumping
chamber 154 and the delivery spring chamber 146. The orifice 140 is
open to allow reagent to flow through the delivery valve element
138 even when the delivery valve element 138 is seated on the
seating surface 136 of the pole element 70. When the delivery valve
element 156 has just closed on the seating surface 136, reagent may
flow into the pumping chamber 154 from the bore 42 of the
connecting pipe 22 so that the pressure of reagent in the bore 42
of the connecting pipe 22 decays rapidly. This causes the nozzle
valve 168 to close more rapidly than would be the case if the
orifice 140 were not present. Furthermore, when delivery of reagent
is not required, for example when the engine is off, any residual
pressure that may be present in the bore 42 of the connecting pipe
22 decays by flow of reagent through the orifice 140. This ensures
that reagent does not undesirably seep through the dispenser 24 and
into the exhaust pipe 28.
[0070] The reagent in the dispenser 24 or in the bore 42 of the
connecting pipe 22 may boil due to an increase in temperature at
the dispenser 24, for example when the engine is switched off and
the flow of reagent through the dispenser 24, which tends to cool
the dispenser 24, ceases. The residual heat in the exhaust system
tends to cause the temperature of the dispenser 24 and the
connecting pipe 22 to rise. Should boiling occur, the orifice 140
allows reagent to flow back into the pump 20 from the bore 42 of
the connecting pipe 22 to avoid a build up of pressure within the
dispenser 24 or the connecting pipe 22.
[0071] In operation, the components within the pump 20 have a
tendency to heat up due to resistive heating of the coil 104 and
magnetic heating due to eddy currents within the armature 128, the
casing 68 and the pole element 70. The reagent flowing through the
annular chamber 112 between the inner pole piece 92 and the coil
former 106 acts to cool the coil 104 and the other components
within the pump 20, because heat is transferred into the reagent
and transported out of the pump 20. The surface area of the annular
chamber 112 exposed to the inner pole piece 92 and the coil former
106 is as large as conveniently possible so as to maximise the heat
transfer from the components of the pump 20 to the reagent.
[0072] The heat transferred to the reagent from the components of
the pump 20 can additionally serve to prevent the reagent from
freezing in cold environmental conditions. To this end, the current
applied to the coil 104 may be controlled to ensure that sufficient
heat is transferred to the reagent to prevent freezing of the
reagent. For example, the application of current to the coil 104
may be maintained for a short time after the plunger 126 has
reached the end of its pumping stroke, so that heat generation
within the pump 20 continues for an extended time and heating of
the reagent is prolonged. Under normal conditions, when the ambient
temperature is such that the reagent is unlikely to freeze, the
current to the coil 104 is switched off at or just prior to the end
of the pumping stroke of the plunger 126.
[0073] When the pump 20 is not operational, for example when the
engine is switched off, the temperature of the residual reagent in
the pump 20 will decrease and will eventually reach the ambient
temperature. Should the reagent freeze within the pump 20, a direct
current can be applied to the coil 104 by way of the power supply
to cause resistive heating of the coil 104. Heat from the coil 104
is dissipated into the frozen reagent by conduction through the
coil former 106 and through the other components of the pump 20 so
that the reagent melts. Alternatively, a large, rapidly pulsing
current can be applied to the coil 104 by the power supply. The
frequency of the pulses is sufficiently high, and the length of
each pulse significantly short, that the resulting magnetic force
on the armature 128 is insufficient to cause movement of the
plunger 126. However, significant eddy currents are induced in the
components within the electromagnetic circuit of the pump 20, and
these eddy currents cause the temperature of the components to
increase and hence encourage rapid melting of the frozen reagent.
In either case, any frozen reagent in the dispenser 24 and the bore
42 of the connecting pipe 22 is melted by heat from the exhaust
pipe 28 or by heat originating from the pump 20 and conducted
through the connecting pipe 22.
[0074] The annular chamber 112 between the inner pole piece 92 and
the coil former 106 is small in the radial direction, so that the
volume change that would occur should the reagent freeze in the
chamber 112 does not give rise to a significant radial stress. In
this way, damage to the components of the pump 20, and in
particular the coil 104, is avoided.
[0075] Movement of the armature 128 is damped by the liquid
surrounding the armature 128. For example, when the armature 128
approaches the inner pole piece 92, the liquid must be displaced or
squeezed out of the space between the armature 128 and the inner
pole piece 92. The resistance of the liquid to displacement acts to
decelerate the armature 128 before it comes into contact with the
inner pole piece 92, thus minimising mechanical noise and reducing
wear on the armature 128 and the inner pole piece 92. Deceleration
of the armature 128 also occurs as the armature 128 approaches the
lift stop 90 after de-energisation of the coil 104.
[0076] The volume of liquid delivered by each pumping stroke of the
plunger 126 can be adjusted during manufacture of the pump 20, for
example to ensure that the pump 20 dispenses an accurately known
volume of reagent with each stroke, and to compensate for
manufacturing tolerances. Typically, the volume of liquid delivered
by each pumping stroke of the plunger 126 is between 1 and 6
mm.sup.3, and in a given pump the volume delivered is adjustable
through approximately 1 mm.sup.3. However, larger or smaller
delivery volumes or adjustment ranges could be provided.
[0077] In one method of adjusting the volume of liquid delivered
with each pumping stroke, the position of the armature 128 on the
plunger 126 is changed to adjust the length of the plunger 126
available to slide within the bore 124 of the inner pole piece
92.
[0078] Because the position of the filling ports 156 is fixed with
respect to the inner pole piece 92, the effective change in volume
of the pumping chamber 154 during a pumping stroke of the plunger
126 is determined by the position of the plunger 126 at the limit
of its pumping stroke, when the armature 128 rests against the
inner pole piece 92. The change in volume of the pumping chamber
154 is not significantly affected by the position of the plunger
126 at the end of its return stroke. This arises because, when the
filling ports 156 are open, liquid may flow away from the pumping
chamber 154 to compensate for movement of the plunger 126. Only
when the filling ports 156 close to isolate the pumping chamber 154
does the plunger 126 act to significantly increase the pressure in
the pumping chamber 154. In this way, moving the plunger 126 in a
downstream direction with respect to the armature 128 causes the
volume of liquid delivered with each pumping stroke to
increase.
[0079] Because the armature 128 is an interference fit on the
plunger 126, the plunger 126 can be forced to slide through the
armature 128 by pressing on the upstream end 130 of the plunger
126. During manufacture, the upstream end 130 of the plunger 126 is
accessible when the inlet connector 66, the lift stop 90 and the
filter 80 are not fitted to the pump 20. Conveniently, an adjusting
fixture (not shown), comprising a body substantially similar to the
inlet connector 66 and a pushrod accommodated within a bore of the
adjusting fixture, is screwed into the casing 68 of the pump 20 in
place of the inlet connector 66. The pushrod is arranged to push
against the upstream end 130 of the plunger 126 on application of a
force to the pushrod so as to adjust the position of the plunger
126 with respect to the armature 128. The adjusting fixture
includes an inlet passage to supply liquid to the pump 20 for
testing the pump 20 during adjustment.
[0080] The amount of adjustment required is determined by
energising the coil 104 of the pump 20 so as to dispense a volume
of reagent, measuring the quantity of reagent, and determining the
deviation of the dispensed volume from the desired volume. If
required, a threaded blind bore (not shown) could be provided in
the upstream end 130 of the plunger 126 to mate with a threaded end
of the pushrod to allow the plunger 126 to be adjusted in either
direction with respect to the armature 128.
[0081] Although the position of the plunger 126 at the end of its
return stroke does not significantly affect the volume of reagent
delivered per pumping stroke, this position does determine what
fraction of the pumping stroke occurs before the filling ports 156
close. Since the armature 128 and the plunger 126 accelerate during
at least a first part of the pumping stroke, the plunger movement
that occurs before the filling ports 156 close influences the
acceleration of the plunger 126 at the instant when the filling
ports 156 close, and subsequently the rate at which the pressure
rises within the pumping chamber 154. The behaviour of the pressure
rise in the pumping chamber 154 influences the dosing
characteristics of the pump 20, for example the output flow
rate.
[0082] It is therefore contemplated that the position adopted by
the plunger 126 at the end of its return stroke may be determined
and adjusted during manufacture of the pump 20 to ensure that the
optimum pressure rise occurs within the pumping chamber 154.
[0083] In one method of adjusting the position adopted by the
plunger 126 at the end of its return stroke, the inlet connector 66
is screwed into or out of the casing 68 so as to adjust the
position of the lift stop 90 with respect to the casing 68. The
position of the lift stop 90 determines the position of the
armature 128, and hence the position of the plunger 126, at the end
of the return stroke.
[0084] The optimum position for the lift stop 90, and hence the
inlet connector 66, is determined by energising the coil 104 of the
pump 20 and measuring the pressure of the liquid output from the
pump 20 as a function of time. The measured pressure-time
characteristic is compared to a reference function comprising a
desired pressure-time characteristic, and the position of the inlet
connector 66 is adjusted to compensate for any deviation between
the measured and desired characteristics.
[0085] The electrical inductance of the coil 104 is a sensitive
function of the position of the armature 128. In a second method of
determining the optimum position of the lift stop 90, a target
inductance value is calculated or otherwise determined. The target
inductance value corresponds to the inductance that occurs when the
armature 128 is optimally positioned with respect to the casing 68
when the armature 128 abuts the lift stop 90. The electrical
inductance of the coil 104 is measured while the position of the
inlet connector 66 is adjusted so that the measured inductance
matches the target value.
[0086] Once the inlet connector 66 is in a suitable position, it
may be fixed in position by, for example, friction between the
threads of the inlet connector 66 and the casing 68, or by gluing,
soldering or welding the threads of the inlet connector 66 to the
threads of the casing 68. Alternatively, or in addition, a locking
nut could be used to lock the inlet connector 66 in position.
[0087] In use, the dispenser 24 of the dosing device is situated
within the flow of exhaust gases upstream of an SCR catalyst, so
that the reagent dispensed by the device is carried within the
exhaust gas flow to the SCR catalyst, where the reagent undergoes a
reaction to chemically reduce NO.sub.x within the exhaust gas flow.
The efficiency of the NO.sub.x reduction reaction can be influenced
by the rate at which reagent is dispensed from the dosing device,
and the manner in which the reagent is dispensed. To control these
characteristics, energisation of the coil 104 is regulated as will
now be described.
[0088] The current applied to the coil 104 is controlled by an
engine control unit (not shown) by way of the power supply. The
engine control unit stores parameters such as the volume of reagent
dispensed per stroke of the plunger 126 and the concentration of
the reagent, and monitors signals corresponding to further
parameters such as engine speed, ambient temperature, SCR catalyst
temperature, reagent temperature, exhaust oxygen content, and so
on. The engine control unit determines, based on these parameters,
how the current should be applied to the coil 104 to dispense the
reagent in such a way that the efficiency of the reduction of
NO.sub.x in the exhaust gas is maximised. Typically, the current is
supplied to the coil 104 in pulses, the duration, magnitude and
profile of each pulse being variable to optimise the output of the
pump 20 to suit the operating conditions of the engine at a given
time.
[0089] The reagent temperature is an important parameter, since the
density of the reagent, and hence its volumetric concentration,
varies with temperature. For example, when the reagent is warm, the
density of the reagent, and hence its volumetric concentration,
decreases. If a constant rate of reagent addition is required, the
frequency of the current pulses may be increased to compensate for
the decrease in concentration.
[0090] To determine the reagent temperature, the engine control
unit is arranged to measure the resistance of the coil 104 of the
pump 20. Because the coil 104 is cooled by the reagent, the
temperature of the coil 104 is closely related to the temperature
of the reagent. For example, when the reagent is cold, the rate of
heat transfer to the reagent from the coil 104 will be high and the
coil 104 will be cooled to a greater extent than if the reagent
were warm. The resistance of the coil 104 is a function of its
temperature, so that the reagent temperature can be calculated from
the resistance measurement.
[0091] A second embodiment of the present invention will now be
described with reference to FIG. 6. The second embodiment is
similar in form and operation to the first embodiment, and like
features share like reference numerals. Only the differences
between the second and first embodiments will be described.
[0092] The reagent dosing device of the second embodiment comprises
a pump 20a, a connecting pipe 22a, and a dispenser 24a. The
connecting pipe 22a comprises a straight tube 40a, so that the
dispenser 24a is coaxial with the pump 20a. A nozzle of the
dispenser 24a comprises a region of the tube 40a shaped into an
outwardly-directed nozzle flange 163a, and a nozzle bore 166 to
house a nozzle valve 168.
[0093] A pole element 70a comprises an inner pole piece 92a, an
outer pole piece 198, a tubular projection 96a and a flange 94a.
The outer pole piece 198 comprises an upstream end region 200
having a wall thickness less than the wall thickness distant from
the end region 200 to define a shoulder 202. The casing 68a
comprises an upstream portion 72a similar to the upstream portion
of the casing of the first embodiment, and an outwardly directed
flange 76a with a bevelled edge 204. The flange 76a of the casing
68a abuts the shoulder 202 of the outer pole piece 198. The end
region 200 of the outer pole piece 198 is bent over the bevelled
edge 204 of the flange 76a, for example by crimping or pressing
during manufacture, so as to hold the casing 68a against the
shoulder 202 of the outer pole piece 198.
[0094] A jacket 44a envelopes the tube 40a of the connecting pipe
22a and partially envelopes the dispenser 24a and the pump 20a. The
jacket 44a is fixed to and sealed against the pole element 70a of
the pump at a first seal 48a close to the upstream end of the pole
element 70a of the pump 20a, and is fixed to and sealed against the
dispenser 24a at a second seal 50a at a rim of the nozzle flange
163a.
[0095] In use, the device is mounted in a wall of an exhaust pipe
28a having an angled port 206. The port 206 comprises a first
portion 208 internal to the exhaust pipe 28a and a second portion
210 external to the exhaust pipe 28a. The first and second portions
208, 210 are tubular and coaxial and are arranged to accommodate
the connecting pipe 22a and dispenser 24a of the device within a
bore 212 of the port 206. The axis of the port 206 lies at an acute
angle to the axis of the exhaust pipe 28a.
[0096] An enclosed compartment 46a is therefore defined between the
pump 20a, the tube 40a, the nozzle flange 163a and the jacket 44a.
The compartment 46a is evacuated so as to minimise the heat
transfer from the exhaust pipe 28a and the port 206 to the reagent
present in the pump 20a, the bore 42a of the connecting pipe 22a
and the dispenser 24a, in use.
[0097] The port 206 further comprises an air inlet 214 comprising a
tubular passage projecting radially from the second portion 210 of
the port 206. A bore 216 of the air inlet 214 is in communication
with the bore 212 of the port 206. The jacket 44a is a clearance
fit in the port 206, except for an engagement region 218 downstream
of the air inlet 214 of the port 206. At the engagement region 218,
the diameter of the jacket 44a is enlarged to form a gas-tight
interference fit in the port 206. The clearance between the jacket
44a and the port 206 defines an air gap 220.
[0098] In use, the air inlet 214 of the port 206 is connected to an
air source (not shown), such as an air bleed taken from an inlet
passage of the engine downstream of a compressor wheel of a
turbocharger. Air therefore enters the air gap 220 and flows within
the air gap 220 before discharging into the exhaust gas flow. The
flow of air in the air gap 220 serves to cool the connecting pipe
22a and the dispenser 24a so as to minimise the heating of the
reagent due to the hot exhaust pipe 28a and exhaust gases.
[0099] It will be appreciated that many modifications of the
embodiments described above lie within the scope of the present
invention. In particular, the components within the pump and their
arrangement may differ from those components and arrangements
previously described.
[0100] Features may be provided to modify or optimise the damping
effect of the liquid on the movement of the armature. For example,
the inner pole piece and/or the lift stop may have conical or
tapered portions to meet with the armature. Similarly, the armature
may have a conical or tapered portion to meet with the inner pole
piece and/or the lift stop. The armature, the inner pole piece
and/or the lift stop may include drillings or slots to modify the
flow of liquid around the armature. To further reduce mechanical
noise, the lift stop may be made from a polymeric or elastomeric
material.
[0101] The filter need not be provided within the pump, for example
if the liquid entering the pump is pre-filtered in a supply system.
As an alternative or in addition to a mesh filter, the filter may
incorporate a magnet, for example a magnetic ring or disc, to
scavenge magnetic particles from the liquid flow. In particular,
solids with a high iron content, which can initiate corrosion of
stainless steel components within the pump, will be trapped by the
magnet before entering the electromagnetic structure of the pump
where they might otherwise be attracted to the magnetised
components.
[0102] The inlet connector may be an interference or push fit
within the casing, in which case neither the inlet connector nor
the complementary portion of the casing would carry threads. In
this case, adjustment of the position of the inlet connector, to
adjust the position of the lift stop and hence the position of the
plunger at the end of its return stroke, would be effected by
sliding the inlet connector within the casing.
[0103] In the described embodiments, the pump connector is formed
integrally with the tube of the connecting pipe. It will be
appreciated that the pump connector could instead be formed as a
separate component. For example, the pump connector could carry
threads to engage with threads carried on the tube or the jacket of
the connecting pipe. The pump connector could alternatively be
formed integrally with the pole element.
[0104] As previously described, the density of the reagent, and
hence the volumetric concentration of the reagent, is a function of
the reagent temperature. The pump may be arranged to automatically
compensate for changes in ambient temperature so that, for example,
as the temperature increases and the reagent concentration
decreases, the volume of liquid dispensed with each pumping stroke
increases.
[0105] For example, the plunger may be made from a material with a
lower coefficient of thermal expansion than the material from which
the inner pole piece is made. When the temperature of the pump
increases, the inner pole piece and the plunger expand, but the
dimensions of the inner pole piece expands relatively more than the
dimensions of the plunger. As a result, the volume of the pumping
chamber increases to counteract the decrease in concentration of
the reagent with temperature. Similarly, the volume of the pumping
chamber decreases when the temperature of the pump falls. If the
inner pole piece were manufactured from an iron-based alloy,
suitable materials for the plunger would include ceramics such as
alumina, silicon carbide and silicon nitride. To achieve a large
amount of thermal compensation, the length of the plunger should be
large relative to the distance moved by the plunger on its pumping
and return strokes.
[0106] A composite plunger made from two or more materials could be
used, arranged so that the coefficient of thermal expansion of at
least a part of the plunger in the radial direction is
approximately equal to that of the inner pole piece, while the
effective coefficient of thermal expansion in the axial direction
is significantly lower than that of the inner pole piece. In this
way, the thermal compensation function is achieved, but the radial
clearance of the plunger in the bore of the pole piece does not
change significantly, to avoid excessive leakage of reagent past
the plunger.
[0107] Many reagents, such as urea solutions, are highly corrosive
to commonly-used metallic alloys. The pump may therefore be adapted
to have high corrosion resistance. For example, the material in the
electromagnetic circuit may be made from a magnetic alloy with a
high chromium content and other alloying elements such as
molybdenum. An example of a suitable material is Carpenter Chrome
Core.RTM. 18-FM, a stainless steel supplied by Carpenter Technology
Corporation.
[0108] Components of the pump may also be surface treated in order
to improve corrosion resistance. Surface treatments may also be
employed to improve wear resistance. For example, stainless steel
components may be wholly or partly subjected to a case-hardening
process in which carbon is introduced into the component by
diffusion so as to form chromium carbides close to the surface. A
suitable process, known as Kolsterising.RTM., is provided
commercially by Bodycote International plc. The chromium carbides
increase the surface hardness of the component to improve wear
resistance, and also improve the corrosion resistance of the metal.
Alternatively, or in addition, surface coatings may be applied to
components of the pump. For example, titanium nitride or
diamond-like carbon coatings may be employed to improve wear and
corrosion resistance. Organic coatings, such as parylene coatings,
may be suitable for components which are not subjected to wear, in
use.
* * * * *